Is Promethazine permissible to use? It is often used by alcoholics.
In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
In principle, it is permissible to consume Promethazine [or Phenergan] medicine for medicinal purposes, in the form of tablets, as they do not contain alcohol. As for the oral liquid form, this generally contains alcohol and should be avoided. 
And Allah Ta’āla Knows Best.
Mufti Mubasshir Talha
3 Dhul Qa’dah 1443 / 3rd June 2022
Mufti Tahir Wadee | Mufti Yusuf Ilolvi
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“Promethazine is both an antihistamine and a sedative. It’s often used for hay fever or as a sleep aid. It does cause sedation and drowsiness. When combined with codeine, it is sometimes abused recreational. Promethazine tablets will not contain alcohol, however American cough syrup that contains codeine and promethazine can contain alcohol and is a growing problem in recreational use.”
[Correspondence with Dr Zuhayr Adia MBBS, Pharmacology Expert, 3rd June 2022]
“Promethazine doesn’t have any alcohol in it, it’s a tablet. No other similar problematic ingredients. Common Medicine. Could be anyone really used for a variety of things. Commonly, Hayfever, insomnia, yes. Sometimes in mental health patients can be used to treat some side effects from antipsychotic medication”
[Correspondence with Adil Khan, Pharmacist, Pharmacy Connect, 3rd June 2022]